unkown change has caused marco to become erratic

Using windows 10 with office 2003.

Built quite a complicated set of multiple macros in excel to perform a specific function 3 years ago and installed it onto 2 different computers. Used daily, all has worked fine until this week when one of the computers now generates a runtime error at a specific part of the code but not all the time. The code is identical on both machines but the other one still works perfectly. The source data is downloaded from a website as a csv, opened in excel and the macro sorts the data into a specific order I have searched for differences in the machines with little success. The source data is the same for both machines and no known updates have taken place recently. This is what I have tried so far. .

On the machine it’s failing on;

Rolled back the last windows update.
Firefox update is latest on failed machine and previous on working machine. Have tried download from Opera and Edge but still generally fails.
Checked security settings and all other settings are the same.
Pasted working code over failed code and saved even though it’s the same.


Today I am going to take a copy of excel from the working machine and set the path to it on the non working machine.

I have also visually compared the results of several working and failed sheets but can see no common issue as why one computer works fine and the other fails. It makes no sense at all. I am certain the coding is fine but something is making it fail and then sometimes it just runs fine?? Any suggestions please

What's the error?

IMG_0467.jpg

This is the error but sometimes it works fine. The range is 1-500 lines

This is the offending line of code.

Sub HERMCombaddress2()
Dim X As Integer

For X = 1 To 500
Cells(X, 31).Value = Cells(X, 11).Value & " " & Cells(X, 5).Value
If InStr(Cells(X, 3), "_") > 0 Then
Cells(X, 11).Value = Left(Cells(X, 11), (InStr(Cells(X, 11), "_")) - 1)
End If
Next X
End Sub

I takes the address values in several cells and combines them into one cell. Works perfectly normally but not now.